added requiresSQLCommentHint true

To make Doctrine schema tool able distinguish between this type and string such that it does not try to update the field every time update schema is run.

src/Rhumsaa/Uuid/Doctrine/UuidType.php

@@ -90,4 +90,14 @@ class UuidType extends Type
     {
         return self::NAME;
     }
+    
+    /**
+     * {@inheritdoc}
+     *
+     * @return boolean
+     */
+    public function requiresSQLCommentHint(AbstractPlatform $platform)
+    {
+        return true;
+    }    
 }